borg compact, Ctrl-C, Lock file visibility

## Have you checked borgbackup docs, FAQ, and open GitHub issues?

Yes

## Is this a BUG / ISSUE report or a QUESTION?

I am unsure, feel free to close if nothing actionable comes out of it.

## System information. For client/server mode post info for both machines.

#### Your borg version (borg -V).

Local: borg 1.2.2
Remote: borg1 1.2.3 (on rsync.net)

#### Operating system (distribution) and version.

Local: NixOS 22.11
Remote: ?

#### Hardware / network configuration, and filesystems used.

Local: Thinkpad, ext4
Remote: ZFS I believe

#### How much data is handled by borg?

A ~250G repo

#### Full borg commandline that lead to the problem (leave away excludes and passwords)

I believe what happened was:

* I got a quota warning from rsync.net
* I ran `borg prune` succcessfully
* I started a `borg compact` run.
* It didn’t print something for a while. I noiced in the logs that there is a `--progress` flag.
* Ctrl-C. The command quit without further output (I think)
* Ran `borg compact --progress`. It complained about a lock file.
* Waited for maybe 5 mins or so, in case there is a remote process winding down.
* Lost patience, and used `borg break-lock`
* Ran `borg compact`. Got various error messages. Annoyingly, I already closed the terminal, sorry…
* Ran `borg check --last 1`. Got messages about missing segments and other issues.

A subsequent repair job finished, removed some segments with zeros, and afterwards I could compact, but I have lost some faith in this particular repository, and am now creating a new one.

I assume what happened was that the first `borg compact` was actually still running on the server, and then I started the second one and that broke things.

I understand it’s mostly my fault, but maybe there are still some lessons to be learned from this that will prevent others from making that mistake.

Some ideas that would have helped me not break my setup:

* `borg compress` could run with progress by default, at least if the output is a terminal.

* I would have expected Ctrl-C to kill the server side `borg` reliably, but maybe that was optimistic. I have seen fixed issues related to that; so maybe it is a `borg compact` specific issue? Or I was just unlucky?

Maybe this could be make more robust?

For example, the local `borg` could wait for some kind of explicit confirmation from the other side that it has stopped, and warn loudly if that did not come?

* The lock file could (if it doesn’t already) contain the pid of the process, and the warning about not being able to aquire the lock file could indicate if a process of that pid is running or not (like vim does). Of course with networked file systems, absence of a running process is not a guarantee, but _presence_ is certainly a sign that one should not `break-lock` too easily. Especially useful on `rsync.net` where I cannot log in an run `ps` manually.

If none of these ideas are useful feel free to close this issue, and nevertheless thanks for maintaining borg!